Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Ore Irradiator
This invention relates to apparatus for detecting the
presence of a selected substance in ores by neutron
activation analysis, for example ~he gold content of
gold-bearing ores.
A practical gold ore sorting plant needs to be able to
process several tonnes of ore an hour, and hence must use a
rapid analytical technique. A suitable technique is
neutron activation analysis using the reaction 197Au
(n, n y ) 197m~u to activate gold present in a
lump of ore, the 197mAu nuclides so produced
decaying ~ith a half-life of about 7.8 seconds, with the
emission of ~ -rays of energy 279 keV. British Patent
Specifications Nos. 2 055 465A and 2 101 304A describe
apparatus for sorting gold bearing ores in which lumps
of ore are activated by the above reaction, the ~ -rays
emitted subsequently being detected and analysed to
assess the gold content of the ores.
According to the present invention there is provided
an irradiator for irradiating lumps of ore for detecting
the presence of a selected substance in the lumps, the
irradiator comprising a plurality of ducts arranged in a
close-packed array, means for rotating each duct about its
longitudinal axis, and a plurality of neutron sources for
irradiating the ducts and located in spaces between the
ducts so that each duct is adjacent to at least one
source.
Preferably, the ducts are arranged to be upright in
use. Each neutron source may comprise a target arranged to
be bombarded by a high energy particle beam, the particle
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beams being incident on the targets in a direction parallel
to the longitudinal axes of the ducts and provided by a
common particle accelerator.
Some of the ducts are desirably adjacent to and
thereby irradiated by more than one neutron source, and
these ducts may be of greater cross-sectional area than
those ducts which are adjacent to only one neutron source.
Conveniently, the number of ducts is six and the
number of neutron sources three, and the longitudinal axes
of the ducts may be located approximately at the vertices
of an equilateral triangle and at the mid~points of the
sides of the triangle.
In the preferred embodiment of the invention for
detecting the presence of gold in gold-bearing ores, each
neutron source comprises a lithium target arranged to be
bo~barded by a beam of high energy protons so as to produce
neutrons of energy between 0.5 and 3.0 ~leV.
The invention will now be further described by way of
example only and with reference to the accompanying
drawings, in which:
Figure 1 is a flow diagram of a gold ore sorting
apparatus including an irradiator
according to the invention; and
Figure 2 is a cross-sectional representation of the
irradiator of Figure 1.
Referring to Figure 1, a gold ore sorting apparatus
comprises a rock crusher and classifier 2 to which mined
ore is supplied, in which the ore is crushed into lumps and
divided into two streams of lumps corresponding to mesh
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sizes OL about 80mm and 60mm respectively, while lumps
smaller than mesh size about 35mm are rejected. Both
streams of lumps are passed through an irradiator 4 to be
described in more detail later, and then all the lumps are
caused to pass a ~~ray detector assembly 6 arranged to
detect ~ -rays having an energy of 279 keV arising from
the decay of 197mAU nuclides and so signifying the
presence of gold in the lumps of ore. Each lump of ore is
interrogated individually by the detector assembly 6 to
establish whether its gold content lies above or below some
predetermined concentration. The critical concentration is
typically in the range 0.5 to 5 parts per million (ppm),
and might for example be set at 1 ppm~ Each lump of ore is
then passed into a sorter 8 arranged by means of a cable 7
lS to respond to signals from the detector assembly 6, and to
sort each lump of ore into one of two outlet streams
depending on whether the gold concentration in the lump
lies above or below the predetermined concentration.
The crusher and classifier 2 and the sorter 8 may be
of types well ~nown in the art, while the detector assembly
6 may be as described more fuily in the aforementioned
specifications to which reference may be made, the crusher
and classifier 2, the sorter 8 and the detector assembly 6
not being the subject of the invention.
Referring to Figure 2, the irradiator 4 comprises a
close packed array of three outer tubular steel cylinders
12 and three inner tubular steel cylinders 14, with their
longitudinal axes upright, the axes of the three outer
cylinders 12 being at the corners of a notional equilateral
triangle, and the axes of the three inner cylinders 14
being near the mid points of the sides of the triangle.
Each of the inner cylinders 14 is of internal diameter 32cm
and each of the outer cylinders 12 is of internal diameter
26cm. The outer cylinders 12 and the inner cylinders 14,
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although close packed, are not contiguous, and each is
caused to rotate about its longitudinal axis in a clockwise
sense by a drive from an electric motor (not shown).
The outer cylinders 12 and the inner cylinders 14
adjacent thereto define between them three spaces 15 in
which respective neutron sources 16 are situated in a
horizontal plane at an intermediate position along the
length of the outer cylinders 12 and the inner cylinders
14. Each neutron source 16 comprises a lithium target, at
the end of a respective evacuated flight tube (not shown),
the three flight tubes emerging via a beam splitter (not
shown) from a single proton accelerator (not shown), and
the end portion of each flight tube extending parallel to
the axes of the cylinders 12, 14. Flight tubes, beam
splitters and accelerators are well known in the art, and
further details will not be given.
The outer cylinders 12, the inner cylinders 14 and the
neutron sources 16 are located within a cylindrical chamber
20 defined by a concrete radiation shield 22 which is
sufficiently thick to be substantially impervious to
y-rays and to neutrons and far enough away so as not to
generate an intense field of interfering low energy
neutrons. A typical distance is lOOcm.
In operation of the irradiator 4, the streams of ore
from the crusher and classifier 2 (see Figure 1) are passed
through the outer cylinders 12 and the inner cylinders 14,
the stream of larger lumps of ore being passed through the
inner cylinders 14. The accelerator is energised to cause
a beam of protons of energy 4.5 MeV to bombard each lithium
target, the proton beam being moved around the surface of
the target to avoid localized overheating. Fast neutrons
of energy between about 0.5 MeV and 2.8 MeV are produced by
the reaction 3 Li (p, n) 7 Be, and irradiate
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the lumps of ore in the adjacent outer cylinders 12 and
inner cylinders 14. The neutron flux decreases with
distance from the sources 16, hence to provide as uniform
an irradiation of the ore lumps as possible the outer
cylinders 12 and the inner cylinders 14 are rotated about
their respective longitudinal axes so that each lump of ore
traverses the high intensity region of the neutron flux
from one of the sources 16 twice, as it passes along the
cylinder 12, 14.
If any gold is present in a lump of ore it will be
activated by the reaction 197AU (n, n y ) 197mAu
the cross-section for which is a maximum for neutrons of
energy about 2.5 MeV. Fast neutrons of energy below 2.8
r~eV are capable of bringing about this activation, but have
insufficient energy to bring about activation by (n, p)
reactions of other elements which are likely to be present
in the ore, such as aluminium and silicon.
The irradiator 4 enables a greater throughput of ore
to be irradiated than the irradiators described in the
aforementioned specifications and also enables larger lumps
of ore to be irradiated. This is because each of the
sources 16 can produce a total neutron flux of about
1012 neutrons/second, about ten times greater than the
neutron fluxes available from the sources described in the
aforementioned specifications, and because the use of two
sources 16 to irradiate each of the larger, inner cylinders
14 means that substantially uniform irradiation and
activation of all the lumps of ore in each inner cylinder
14 is achievable with cylinders of larger diameter than can
be satisfactorily irradiated with a single neutron source.
It will be appreciated that the uniformity of
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irradiation of lumps of ore in the three inner cylinders 14
may be still further improved by the provision of a fourth
neutron source at the position marked A in Figure 2 between
the three inner cylinders 14.
The inner cylinders 1~ and the outer cylinders 12 may
have diameters differing from the values given. However to
avoid interfering excitations brought about by intermediate
energy neutrons all the cylinders are preferably of
diameter less than about 5Ccm, while to ensure uniform
irradiation of the lumps of ore those cylinders which are
adjacent to only one neutron source are preferably of
diameter less than 35cm. It will also be appreciated that
the number of cylinders need not be six, and that they may
be disposed in a different close-packed array.
It will be understood that although the sources 16
have been described as comprising lithium targets onto
which generally vertical beams of protons are incident from
a common accelerator, each target may be bombarded by a
particle beam from a separate accelerator, and if desired
the beams may be incident in generally horizontal
directions through the spaces between the adjacent outer
cylinders 12 and inner cylinders 14. It will also be
understood that alternative neutron sources such as those
utilizing a D (D, n) 3He reaction may be used as long as
the neutrons produced are of energies less than about
3 MeV.
